You may need to try to set the remaining and not allocated partitions as GPT (GUID Partition Table) using Windows's Disk Management and see if the operating system is able to allocate the sapce after that.
The DG45ID has issues with GPT and UEFI (even with Win 7 64 bit). Technically if UEFI is enabled in the BIOS and Win 7 (64 bit) OS is installed, it should be possible to boot off > 2 TB arrays but the Matrix technology doesn't allow this for some reason. I bought 4 x 1 TB HDDs and was hoping to have RAID 5 with this board but because of the above I'm forced into RAID 0+1 if I want just the one drive showing up in Windows Explorer. Just as well I didn't go for 1.5 or 2 TB drives. The only way I can see the OP making full use of these drives is by having a non-RAID boot drive for the PC.
I have four 2TB Hitachi 7200rpm SATA drives that are setup in a dual volume (RAID 5 boot, RAID 0 scratch) array. This DG45ID set-up with the latest July 2010 BIOS reliably boots Windows 7 Ultimate 64-bit which has to be set to 2047GB or below in the BIOS Matrix RAID Manager for the utility to "grant it" bootable status. I could have easily set the boot volume to RAID 0, RAID 1, but in this case I did RAID 5/0. My set-up is exactly as depicted in the Intel Rapid Storage Technology Review webpage picture (http://www.intel.com/design/chipsets/matrixstorage_sb.htm#store).
I am well aware of the limitations of Host RAID versus a stand-alone RAID card, but I figured with a Q9650 processor, it should be able to handle it; which, it does quite well. ATTO Disk Benchmark for the RAID 5 boot volume gives 344Mb/s read and 70Mb/s write. ATTO Disk Benchmark for RAID 0 scratch volume was 422Mb/s read and 417Mb/s write. Yes, I know this is slow compared to SSDs. However, given parity calculations, not having to worry about wear-leveling/trim support, and native RAID 5 reliability, not to mention $87/drive superdeal cost (Fry's Electronics), simple economics wins again. The Matrix Raid ICH10R is rock solid. I had a bad system memory card which caused me BSODs for a repeat Win7 install (long live WinXP Pro SP3!) . My raid volumes and all data (save for windows) were intact! Go Intel! This situation was even after multiple BSODs caused page fille errors and caused my computer to power off sporadically until I nailed it down to a nasty little memory card that was the culprit. Replaced it, problem solved.
On the allocation issue, I have still not been able to access the remaining hard drive space beyond the 2TB limit for the RAID 0 scratch volume. That is, there are four 2TB drives for 8TB total. After divisioning to RAID 5/0 volume, it left about 5.7Gb to work with. The first 2047GB was set to the RAID 5 boot volume. Matrix BIOS assigned the rest to RAID 0 volume which could not be further subdivided in the Matrix BIOS (only lets you make two volumes on the RAID array, so you are stuck if you have more than 4TB of useable hard drive space). In Windows 7 Ultimate, boot RAID 5 drive reports at 1.99TB and the scratch RAID 0 drive as 1.99TB. Compmgmt.msc command brought up the Disk Management tool, boot RAID 5 drive reported at 2046.90Gb with 100mb system reserve for win 7. RAID 0 drive reported at 2048.00Gb. Windows recognizes another 2674.71Gb of unallocated drive space that I cannot do anything with. I thought maybe I can create another logical drive within windows below the 2TB limit, but no go. This issue seems to be one for both Windows and Intel. Intel on the front end could solve this problem by making a bios update that allows more than 2 volumes to be created on a single RAID array or increase the 2TB ceiling for drive volume recognition. Windows on the back end could make some update in the Disk Management utility to allow allocation of the Windows-recognized, but unallocatable drive space. These types of updates would not be specific to this board but to the ICH10R processes, it would seem, on any motherboard.
So if your RAID 0 is going to be non-bootable use RST in windows to use max space left for a array and use a GPT partition so you can go 2TB+ on it.
XP will not support this.
More then two arrays across the same disks in a RAID will not likely get supported but support for making a array bigger then 2TB is supported as long as you use RST in windows and use GPT and is not going to be used as a boot volume.
Peter UK pointed me in the right direction. So after some digging on GUID partition table (GPT) disk partitioning system on the Microsoft website, it turns out that all one has to do is make sure there is no data at all on the second non-boot RAID volume until one gets into Windows. In Disk Manager (accessed using the compmgmt.msc command), it will then allow you to right click on the volume in question and delete it, if need be, or create a new volume (limitless in theory, but limited to 128 logical drive partitions in Windows, see: http://www.microsoft.com/whdc/device/storage/gpt-on-x64.mspx). The right-click option to delete/create does not appear clickable (grayed-out) if there is ANY data on the volume in question. THerefore, in this case, RST in the Windows environment was not even needed per se as the RAID volumes were set up even before Windows (in this case, Win 7, 64-bit) was installed. GPT commands were also not needed as all that was left to do was right click on the RAID 0 volume in question and format it to full capacity. Windows automatically set up the GPT expanded RAID volume (in this case, a RAID 0 scratch non-bootable drive). The user-opted GPT function in Windows would come into play if one wanted to convert an already existent volume into a logical one that standard MBR could not do above the 2TB limitation for drive space. In my case, I was not converting a volume, but rather removed all data and re-created the volume bypassing the need for GPT conversion from MBR. That Brit, Peter, is smart.